1. Field of the Invention
The present invention relates to a method of producing an ultrasonic flowmeter, an ultrasonic flowmeter produced by the method, and a fluid controller having the ultrasonic flowmeter, which ultrasonic flowmeter is used in fluid transportation in various industries such as chemical works, semiconductor manufacture field, food processing field and biotechnology field, which propagates an ultrasonic vibration through a fluid and measures a flow velocity or flow rate of the fluid from a difference between ultrasonic wave propagation time from an upstream side of the flow and ultrasonic wave propagation time from a downstream side of the flow. The present invention particularly relates to a method of producing an ultrasonic flowmeter, an ultrasonic flowmeter produced by the method, and a fluid controller having the ultrasonic flowmeter, which ultrasonic flowmeter is suitable for measuring a micro flow rate and the flow rate of a slurry fluid or especially the CMP slurry fluid used in the semiconductor field.
2. Description of the Related Art
Ultrasonic flowmeters for measuring a flow velocity or flow rate of a fluid flowing in a measurement pipe from a difference in ultrasonic wave propagation time are generally classified into two types.
In a first type of ultrasonic flowmeter, flow passages are connected to both ends of a linear measurement pipe so that the flow passages are at generally right angle to the measurement pipe, and ultrasonic transceivers are disposed on an upstream side and a downstream side of the measurement pipe so that the ultrasonic transceivers face each other across the measurement pipe. In the ultrasonic flowmeter, an ultrasonic wave transmitted from the upstream ultrasonic transceiver is propagated through a fluid in the measurement pipe and received by the downstream ultrasonic transceiver. Instantaneously after that, an ultrasonic wave transmitted from the downstream transceiver is propagated into the fluid in the measurement and received by the upstream ultrasonic transceiver (see Japanese Unexamined Patent Publication Nos. 2000-146645, 2006-337059, 2007-58352, etc.). In the process, a difference between the ultrasonic wave propagation time from the upstream ultrasonic transceiver to the downstream ultrasonic transceiver and the ultrasonic wave propagation time from the downstream ultrasonic transceiver to the upstream ultrasonic transceiver is used to measure the flow velocity of the fluid in the measurement pipe and measure the flow rate.
In a second type of ultrasonic flowmeter, two ultrasonic transceivers are disposed on transmitting bodies mounted on outer peripheral portions of a linear measurement pipe, respectively. In the ultrasonic flowmeter, an ultrasonic wave transmitted from one of the ultrasonic transceivers is propagated into a fluid in the measurement pipe through the transmitting body and a wall of the measurement pipe, propagated obliquely with respect to a flowing direction of the fluid in the measurement pipe while being reflected on the pipe wall of the measurement pipe, and received by the other ultrasonic transceiver. Instantaneously after that, the transmitting side and the receiving side are switched, and, similarly to above, an ultrasonic wave transmitted from one of the ultrasonic transceivers is received by the other ultrasonic transceiver (see Japanese Unexamined Patent Publication Nos. 2005-188974, 2008-275607, 2011-112499, etc.). In the process, like the first type of the ultrasonic flowmeter, a difference between the ultrasonic wave propagation time from the upstream ultrasonic transceiver to the downstream ultrasonic transceiver and the ultrasonic wave propagation time from the downstream ultrasonic transceiver to the upstream ultrasonic transceiver is used to determine the flow velocity of the fluid in the measurement pipe and measure the flow rate
In the first type of the ultrasonic flowmeter, bent portions are formed on both end portions of the measurement pipe. Therefore, especially in a case where a fluid flowing in the measurement pipe is a slurry, the slurry is deposited and fixed to the bent portions, and propagation of the ultrasonic vibration is hindered, thus causing a problem that accurate measurement of the flow rate is not possible. On the contrary, the second type of the ultrasonic flowmeter has an advantage that the above-mentioned problem is unlikely to happen since it is not necessary to form bent portions on both end portions of the measurement pipe.
However, in the second type of the ultrasonic flowmeter, it is necessary to provide the transmitting bodies on the outer peripheral portion of the measurement pipe. In a case where the transmitting bodies fabricated in a process different from the measurement pipe fabricating process are later mounted to the measurement pipe by an adhesive, welding, etc., it is likely that positions of the transmitting bodies with respect to the measurement pipe and a distance between the transmission bodies vary depending on proficiency of an operator, thus causing deterioration of measurement accuracy. Further, factors such as an amount of adhesive applied, drying time of the adhesive, uniformity of application of the adhesive, etc., cause variation in performance of the ultrasonic flowmeter, and therefore need to be controlled in order to ensure performance of the ultrasonic flowmeter. In addition, in a case where a small-diameter measurement pipe is used, a problem occurs that it is difficult to assemble the measurement pipe and the transmitting bodies. It is not necessary to use an adhesive when the measurement pipe and the transmitting bodies are formed integrally with each other by injection molding. However, it is necessary to provide a draft in an inner diameter of the measurement pipe, which makes a flow velocity of a fluid in the measurement pipe non-constant. Therefore, forming the measurement pipe and the transmitting bodies integrally with each other is not suitable especially for fabricating a small-diameter measurement pipe. As a result, when fabricating the transmitting bodies and the measurement pipe integrally with each other, cutting work is often used.
However, with the cutting work, it is especially difficult to fabricate a measurement pipe having a small pipe diameter, and it is also difficult to control quality of an inner peripheral surface of the measurement pipe. Further, microasperity is formed on the inner peripheral surface of the measurement pipe, and microscopic bubbles are thus easily adhered to the inner peripheral surface of the measurement pipe. Surfaces of the microscopic bubbles reflect an ultrasonic vibration, thereby causing a decrease in output signal strength and deterioration of measurement accuracy especially in the second type of the ultrasonic flowmeter in which the ultrasonic vibration is propagated while being reflected within the measurement pipe.
In order to solve the problem of the microscopic bubbles inside the measurement pipe, Japanese Unexamined Patent Publication No. 2012-42243 suggests a straight-pipe type ultrasonic flowmeter in which, as shown in FIG. 10, a measurement portion 103 provided in a measurement space 102 of a housing 101 includes a straight pipe member 104 for measurement through which a fluid for measurement flows, and a pair of transducers 105 disposed on an outer periphery of the pipe member 104 at a given interval in an axis direction. A diameter-reduced portion or a bubble-crushing portion 106 is provided on a downstream side of the pipe member 104, thereby crushing small bubbles, which are generated when a flow rate is small and are likely to gather near an inner wall surface. However, pressure drop is caused by the diameter-reduced portion provided as the bubble-crushing portion 106, and foreign matters are likely to be adhered to and deposited on the diameter-reduced portion. Further, it becomes difficult for regular-sized bubbles to pass through due to the diameter-reduced portion, which can cause deterioration of measurement accuracy.
Accordingly, it is an object of the present invention to solve the problems of the prior art and to provide an ultrasonic flowmeter with high measurement accuracy, in which transmitting bodies for ultrasonic transceivers to be mounted thereon are formed integrally with a measurement pipe.
In a first aspect, according to the present invention, there is provided a method of producing an ultrasonic flowmeter including a measurement pipe through which a fluid flows, and two ultrasonic transceivers provided on outer side portions of the measurement pipe so as to be spaced apart from each other in an axis direction, which includes steps of: fabricating the measurement pipe in advance; setting the measurement pipe in a mold as an insert; forming two transmitting bodies by insert molding on the outer side portions of the measurement pipe so as to be spaced apart from each other in the axis direction, so that the two transmitting bodies are integral with the measurement pipe; and mounting the two ultrasonic transceivers on the two transmitting bodies, respectively.
In the method of producing the ultrasonic flowmeter, the measurement pipe fabricated in advance is arranged as the insert in the mold, and the transmitting bodies are formed by insert molding on the outer side portions of the measurement pipe so that the transmitting bodies are integral with the measurement pipe. Therefore, it is possible that the measurement pipe and the transmitting bodies are fabricated in different processes, and it is easy to improve smoothness of an inner peripheral surface of the measurement pipe. Thus, the above method can be easily applied to a measurement pipe having a small diameter. Further, in the above ultrasonic flowmeter production method, it is not necessary to integrate the measurement pipe with the transmitting bodies by using an adhesive. Therefore, a problem that performance of the ultrasonic flowmeter can be varied due to use of an adhesive is avoidable, and fabrication of the measurement pipe having a small diameter can be easier. It is also possible to form the transmitting bodies accurately at predetermined positions on the outer side portions of the measurement pipe with almost no variation. As a result, it can be easier to ensure a certain level of measurement accuracy without depending on proficiency of an operator.
In the ultrasonic flowmeter production method, the measurement pipe is preferably fabricated by extrusion molding. Since the inner peripheral surface of the measurement pipe fabricated by extrusion molding has a small surface roughness, it is possible to prevent microscopic bubbles from being adhered to the inner peripheral surface of the measurement pipe. Also, when extrusion molding is used, unlike injection molding, no draft is needed in the inner peripheral surface of the measurement pipe, thus preventing influence of the draft on measurement. Therefore, the above method makes it easier to fabricate a measurement pipe having a small diameter and can be applied to fabrication of a measurement pipe having a wide range of diameters. Further, when the measurement pipe is fabricated, heat is applied once. Therefore, when insert molding is carried out by using the measurement pipe, excellent thermal stability and productivity are obtained because the measurement pipe is heated once when the measurement pipe is fabricated. The arithmetic mean roughness Ra of the inner peripheral surface of the measurement pipe more preferably satisfies a relation of 0 μm<Ra≦0.2 μm. When the arithmetic mean roughness of the inner peripheral surface of the measurement pipe is within the above range, adhesion of microscopic bubbles to the inner peripheral surface of the measurement pipe can be prevented effectively.
Preferably, an inner diameter D of the measurement pipe satisfies a relation of 0.5 mm≦D≦10 mm.
Also, the measurement pipe and the transmitting bodies are preferably made of a same material, and the measurement pipe and the transmitting bodies are more preferably made of a fluorine resin.
In the above-mentioned ultrasonic flowmeter production method, a flow passage or a joint may be formed by insert molding on at least one of an upstream side and a downstream side of the measurement pipe so as to be integral with the measurement pipe.
In a second aspect, according to the present invention, there is provided an ultrasonic flowmeter produced by the above-mentioned ultrasonic flowmeter production method.
In a third aspect, according to the present invention, there is provided a fluid controller including the ultrasonic flowmeter described above, and a control part controlling an instrument in accordance with an output from the ultrasonic flowmeter.
In the ultrasonic flowmeter production method according to the present invention, the measurement pipe and the transmitting bodies are formed by insert molding so as to be integral with each other. Therefore, the transmitting bodies can be formed, integrally with the measurement pipe, at accurate positions of the outer side portions of the measurement pipe without needing proficiency of an operator and use of an adhesive, thereby improving measurement accuracy of the ultrasonic flowmeter. Further, smoothness of the inner peripheral surface of the measurement pipe can be improved, thereby making it easier to restrain adhesion of microscopic bubbles to the inner peripheral surface of the measurement pipe. As a result, the ultrasonic flowmeter with high measurement accuracy, which is unlikely to be affected by the microscopic bubbles, can be achieved.